This invention relates to a quadrature phase demodulator which is used as a counterpart of a quadrature phase modulator and which is capable of operating under a wide input dynamic range. Such a demodulator is particularly useful in a mobile radio telephone network which comprises a base station having a service area and a plurality of mobile stations present in the service area. Each of the base and the mobile stations comprises a quadrature phase modulator and a receiver.
In the quadrature phase modulator, a pair of quadrature-phase carrier signals are modulated by an input signal into a quadrature phase modulated signal. The quadrature phase modulated signal is delivered to a transmitter connected to the quadrature phase modulator. The transmitter transmits the quadrature phase modulated signal to a receiver of another station through a radio transmission path or link. The receiver receives the quadrature phase modulated signal. A quadrature phase demodulator connected to the receiver is supplied with the quadrature phase modulated signal as a demodulator input signal from the receiver. The quadrature phase demodulator demodulates the demodulator input signal to a demodulated signal. The demodulator input signal has a large varying amplitude caused by various radio transmission link conditions. In order to control the amplitude, the demodulator generally comprises an automatic gain control circuit in addition to a phase detector. However, the automatic gain control circuit has a degraded gain control characteristic when the amplitude varies drastically. This is serious in case where the demodulator input signal includes a burst signal. This means that the quadrature phase modulator has a narrow dynamic range and is undesirable for use in demodulating the demodulator input signal having the amplitude which varies drastically.